1. Field of the Invention
The present invention relates to an optical information recording medium that allows recording and reproduction of information signals with high signal quality by irradiating a thin film formed on a substrate with high energy light beams such as laser beams, a method for producing the same, a method for recording and reproducing information thereon and an optical recording/reproducing apparatus.
2. Description of the Prior Art
When a thin film made of a chalcogen material or the like formed on a substrate is heated locally by being irradiated with laser beams while irradiation conditions are adjusted, a phase change between an amorphous phase and a crystalline phase, which have different optical constants (refractive index n, extinction coefficient k), can be effected in the irradiated portion. Utilizing the phase change, a medium for high speed and high density information recording that detects a difference in the amount of reflected light or the amount of transmitted light with respect to light at a specific wavelength as a signal, and a method for utilizing the medium have been under development.
In the phase change recording, modulating laser output between two levels for recording and erasing in accordance with an information signal and irradiating information tracks with the laser output allow new signals to be recorded while erasing existing signals (e.g., Japanese Laid-Open Patent Publication (Tokkai-Sho) No. (JP-A-) 56-145530). This method is advantageous in that a magnetic circuit, which is required for magneto-optic recording, is not required so that a recording-reproducing head can be simplified, and that a rewriting period can be shortened because erasing and recording are performed simultaneously.
In order to perform phase change recording in high density, the following approach is used: A small recording mark is formed by using a light source with a short wavelength for recording or an object lens with high NA (numerical aperture), or the like, so that the linear density in the circumferential direction and the track density in the radial direction of a disk-shaped substrate can improve. Furthermore, "mark edge recording" where information is based on the length of a recording mark has been proposed for higher density in the circumferential direction, and "land and groove recording" where both grooves for guiding laser light and lands between the guide grooves on the substrate are used as recording tracks has been proposed for higher density in the radial direction.
It is important not only to achieve high density, but also to improve an information processing rate, namely, the rate at which information is recorded and reproduced. Research to achieve higher linear velocity for recording and reproduction by rotating a disk at a high revolution per minute without changing the radial position is under way.
Furthermore, a recording medium whose capacity is doubled by laminating a plurality of recordable information layers via separating layers (e.g., JP-A- 9-212917) and layer-identifying means or layer-switching means that selects one of these information layers for recording and reproduction (e.g., JP-A-10-505188) have been proposed.
When high density for recording is pursued excessively, problems such as overwrite distortion or deterioration due to repetition may be caused. In particular, when forming a recording mark in a track, a recording mark that has been recorded in an adjacent track may be erased partially. This phenomenon (hereinafter, referred to as "cross erase") is more significant as the gap between tracks is made smaller in order to improve the recording density in the radial direction. Especially in the land and groove recording, the gap between recording marks in the radial direction is about half of that in the case where information is recorded either in grooves or lands, so that the cross erase becomes significant.
The cross erase is caused by the fact that a laser light spot focused for recording affects a track adjacent to a targeted track for recording information. More specifically, direct heating of the adjacent track by the laser light or indirect heating of the adjacent track by heat conduction from the targeted track is believed to cause the cross erase.
In the case of overwriting by the same laser light, an amorphous portion and a crystalline portion have different optical absorptances, and the latent heat of melting is consumed by the crystalline portion. Therefore, a difference in the end-point temperature between the amorphous portion and the crystalline portion results when they are irradiated with laser light with the same power, so that when overwriting, the mark edge positions are not uniform due to the influence of a signal that was recorded before the overwriting. This leads to an increase in errors (jitters) in the time-axis direction of reproducing signals or deterioration in the erasure ratio. This phenomenon causes a large problem when high linear velocity and high density for recording are pursued, especially when the mark edge recording system is adopted.
In order to solve this problem, it is necessary to equalize the end-point temperatures in the crystalline portion and the amorphous portion when they are irradiated with laser light with the same power. For this end, it is necessary that a light absorptance ratio Acry/Aamo of the absorptance Acry in the crystalline portion to the absorptance Aamo in the amorphous portion is larger than 1.0 when they are irradiated with laser beams with a wavelength .lambda. in order to compensate for the latent heat of melting in the crystalline state. In addition, it is desirable that a reflectance difference .DELTA.R=Rcry- Ramo between the reflectance Rcry in the crystalline portion and the reflectance Ramo in the amorphous portion is large when they are irradiated with laser beams with a wavelength .lambda. in order to obtain a high C/N ratio.
As conventional techniques for raising both Acry/Aamo and .DELTA.R, a three-layered structure without a reflective layer (JP-A-3-113844 and JP-A-5-298748), a reflective layer composed of a material having a low reflectance, or a four-layered structure including a reflective layer having a sufficiently small thickness (JP-A-4-102243 and JP-A-5-298747) have been proposed.
However, the above-mentioned approaches do not necessarily achieve sufficient recording and reproducing characteristics even if both Acry/Aamo and .DELTA.R become large. For example, when the thickness of the reflective layer is small or when the heat conductivity of the reflective layer is low, so-called cooling ability that allows heat generated by light absorption of a recording layer to escape to the reflective layer is insufficient, thus disturbing the change to an amorphous state. This phenomenon occurs significantly in the front end portion of an amorphous mark, and the width of the front end portion of the mark becomes smaller than that of the rear end of the mark, thus causing an imbalance. Furthermore, this phenomenon causes not only the physical dislocation of the mark edge position from the intended position, but also the non-uniformity of the mark width results in significant dislocation of the edge position as an electric signal, thus leading to an increase of jitter. This is detrimental to recording in high density and at a high linear velocity rate.